Memory device having a transistor and one resistant element as a storing means and method for driving the memory device

ABSTRACT

A memory device having one transistor and one resistant element as a storing means and a method for driving the memory device, includes an NPN-type transistor formed on a semiconductor substrate, an interlayer insulating film formed on the semiconductor substrate to cover the transistor in which a contact hole exposing a source region of the transistor is formed, a resistant material in which a bit data “0” or “1” is written connected to the source region of the transistor by a conductive plug or an insulating film, and a conductive plate contacting the resistant material. The memory device exhibits improved degree of integration, reduced current consumption by lengthening a refresh period thereof, and enjoys simplified manufacturing process due to a simple memory cell structure.

This application is a DIVISION of application Ser. No. 10/602,736, filedJun. 25, 2003 now U.S. Pat. No. 6,838,727.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory device and a method fordriving the same. More particularly, the present invention relates to amemory device having a cell that has one transistor and one resistantelement, in which the resistant element acts as a storing means, and amethod for driving the memory device.

2. Description of the Related Art

In general, a memory device, particularly a unit memory cell in whichdata is written in a DRAM (dynamic random access memory), is constructedwith one transistor and one capacitor. The capacitor represents a regionin which data is written, namely, a data storage region. In order toprevent any data loss or data error when writing and reading data, thecapacitor is required to have a certain electrostatic capacity.

As memory devices become more highly integrated, a region occupied by acapacitor in a memory cell becomes smaller. However, the electrostaticcapacity of the capacitor required for storing data remains the same.

To increase the electrostatic capacity of a capacitor in a limitedregion, the region of the capacitor in which electrodes are positionedmust be as large as possible, the distance between the electrodes assmall as possible, and the dielectric material between the electrodes asconductive as possible.

By processing electrodes to have three-dimensional shapes such ascylindrical shapes, the region of the capacitor in which the electrodesare positioned may be enlarged. However, unlike manufacturing capacitorshaving two-dimensionally shaped electrodes, manufacturing a capacitorhaving such three-dimensionally shaped electrodes is difficult due tothe structural complexity of the capacitor. By decreasing the thicknessof the dielectric material, the distance between the electrodes may bedecreased. However, a thin dielectric layer leads to increased leakagecurrent. Using a dielectric material that is highly conductive greatlyincreases the electrostatic capacity of the capacitor compared to usinga thin dielectric film. However, when manufacturing semiconductordevices using highly conductive materials as capacitor dielectrics,etching becomes more complicated and the product price increases becausethe materials used for the electrode are limited to precious metalshaving high etching resistance.

Due to the existing problems, the manufacturing process of a memorydevice using a capacitor as a storing means becomes more complex. As aresult, manufacturing reproducibility and memory device reliability maybe poor, leading to a sharp decrease in yield.

SUMMARY OF THE INVENTION

It is therefore a feature of an embodiment of the present invention toprovide a memory device having an enhanced degree of integration due toa simple memory cell structure of the memory device, simplifying amanufacturing process of the memory device and lengthening a refreshperiod of the memory device to reduce current consumption.

It is another feature of an embodiment of the present invention toprovide a method for driving the memory device.

In one embodiment, the present invention provides a memory deviceincluding a semiconductor substrate, an NPN-type transistor formed onthe semiconductor substrate, an interlayer insulating film formed on thesemiconductor substrate to cover the transistor, in which a contact holeexposing a source region of the transistor is formed, a conductive plugfilling the contact hole, a resistant material in which a bit data “0”or “1” is to be written formed on the conductive plug, a conductiveplate formed on the interlayer insulating film to be contacted with theresistant material. The resistant material is preferably contacted withthe source region of the transistor through the conductive plug.

Additional features and advantages of the invention will be set forth inthe description which follows and, in part, will be obvious from thedescription, or may be learned by practice of the invention.

A first material film through which electrons can tunnel may bepositioned between the conductive plug and the resistant material. Asecond material film through which electrons can tunnel may bepositioned between the resistant material and the conductive plate.Either of the first and second material films may be an n-type polysilicon film, a p-type silicon film, a silicon oxide film or an aluminumoxide film.

The resistant material is preferably an amorphous dielectric filmcapable of trapping electrons during a predetermined time required forstoring data according to predetermined values or directions of avoltage or current. The amorphous dielectric film is preferably asilicon nitride film (Si₃N₄) or an aluminum oxide film (Al₂O₃).

When the resistant material is the silicon nitride film, the conductiveplug is preferably the same material layer as the material layer of thesource region and the conductive plate is preferably an aluminum (Al)plate.

When the resistant material is the aluminum oxide film, the conductiveplug is preferably a gold (Au) plug or a platinum (Pt) plug, and theconductive plate is preferably an aluminum (Al) plate.

A material layer including the conductive plug, the resistant materialand the conductive plate preferably has a thickness that allows chargesused for writing the bit data to tunnel through the material layer. Amaterial layer including the conductive plug, the first material film,the resistant material and the conductive plate preferably has athickness that allows charges used for writing the bit data to tunnelthrough the material layer. A material layer including the conductiveplug, the first material film, the resistant material, the secondmaterial film and the conductive plate preferably has a thickness thatallows charges used for writing the bit data to tunnel through thematerial layer.

In another feature of an embodiment of the present invention, a memorydevice is provided including a semiconductor substrate, an NPN-typetransistor formed on the semiconductor substrate, an interlayerinsulating film formed on the semiconductor substrate to cover thetransistor, in which a contact hole exposing a source region of thetransistor is formed, an insulating film formed on the entire surface ofthe source region exposed through the contact hole, a resistant materialin which a bit data “0” or “1” is written formed on the interlayerinsulating film to be contacted with the entire surface of theinsulating film, and a conductive plate covering the entire surface ofthe resistant material.

A material film, through which electrons can tunnel, may be furtherpositioned between the resistant material and the conductive plate.Here, the material film is preferably an n-type poly silicon film, ap-type silicon film, a silicon oxide film or an aluminum oxide film. Theresistant material is preferably an amorphous dielectric film capable oftrapping electrons during a predetermined time required for storing dataaccording to predetermined values or directions of a voltage or current.The amorphous dielectric film is preferably a silicon nitride film(Si₃N₄) or an aluminum oxide film (Al₂O₃). The conductive plate ispreferably an aluminum (Al) plate.

It is another feature of an embodiment of the present invention toprovide a method for writing bit data of a memory device that includes asemiconductor substrate, an NPN-type transistor formed on thesemiconductor substrate, an interlayer insulating film formed on thesemiconductor substrate to cover the transistor, in which a contact holeexposing a source region of the transistor is formed, a conductive plugfilling the contact hole, a resistant material in which a bit data “0”or “1” is written formed on the conductive plug, and a conductive plateformed on the interlayer insulating film to be contacted with theresistant material, the method including initializing the resistantmaterial and charging the resistant material to write the bit data “0”or “1” therein.

Preferably, the conductivity of the resistant material is enhanced by aforming process. A forming voltage is preferably applied to a drain ofthe transistor.

Preferably, the transistor is on, a bit line voltage (Vb) is applied toa drain region of the transistor and a plate voltage (Vb/2) is appliedto the conductive plate, to write the bit data “0” or “1” in theresistant material.

After the bit data “0” or “1” is written, the transistor is turned offto lengthen a time the bit data is retained.

It is another feature of an embodiment of the present invention toprovide a method for writing bit data of a memory device includinginitializing the resistant material and enhancing the resistance of theresistant material and writing the bit data “0” or “1” therein.

The resistance of the resistant material may be enhanced by dischargingthe resistant material, turning the transistor on and applying a platevoltage (Vb/2) to the conductive plate, or turning the transistor on andapplying a switching voltage (Vs) to the conductive plate.

Preferably, the resistant material, as an amorphous dielectric film, isa silicon nitride film or an aluminum oxide film. When the resistantmaterial is the silicon nitride film, the transistor is turned on, anopposite voltage to a bit line voltage (Vb) is applied to a drain regionof the transistor and a plate voltage (Vb/2) is applied to theconductive plate, to enhance the resistance of the resistant material.When the resistant material is the aluminum oxide film, the transistoris turned on, a different voltage from a bit line voltage (Vb) isapplied to a drain region of the transistor and a plate voltage (Vb/2)is applied to the conductive plate, to enhance the resistance of theresistant material.

It is yet another feature of an embodiment of the present invention toprovide a method for reading a written bit data in a memory devicemeasuring a current flowing from the resistant material and reading thewritten bit data in the resistant material. After a sense amplifier isconnected to the drain region of the transistor, the transistor isturned on and a plate voltage (Vb/2) is applied to the conductive plateto measure the current flowing through the resistant material.

Another feature of the present invention provides a method for reading awritten bit data in a memory device that includes a semiconductorsubstrate; an NPN-type transistor formed on the semiconductor substrate;an interlayer insulating film formed on the semiconductor substrate tocover the transistor, in which a contact hole exposing a source regionof the transistor is formed; a conductive plug filling the contact hole;a resistant material in which a bit data “0” or “1” is written formed onthe conductive plug; and a conductive plate formed on the interlayerinsulating film to be contacted with the resistant material, wherein thebit data which is written in the resistant material is read by measuringa current flowing through the resistant material and reading the writtenbit data in the resistant material. Preferably, after a sense amplifieris connected to a drain region of the transistor, the transistor isturned on and a reading voltage (Vr) is applied to the conductive plateto measure the current flowing through the resistant material.

Accordingly, it is possible to enhance a degree of integration of amemory device by simplifying a memory cell structure and a manufacturingprocess thereof. It is also possible to reduce current consumption ofthe memory device by lengthening a refresh period of the memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will becomemore apparent to those of ordinary skill in the art by describing indetail preferred embodiments thereof with reference to the attacheddrawings in which:

FIG. 1 illustrates a sectional view of a memory device according to afirst embodiment of the present invention;

FIGS. 2 through 4 illustrate sectional views of transformation examplesof the memory device of FIG. 1;

FIG. 5 illustrates a sectional view of a memory device according to asecond embodiment of the present invention;

FIG. 6 illustrates a sectional view for explaining a method for writingbit data “0” by using the memory device of FIG. 1;

FIG. 7 illustrates a sectional view for explaining a method for writingbit data “1” by using the memory device of FIG. 1;

FIG. 8 illustrates a sectional view of showing charge retention of thememory device of FIG. 1;

FIGS. 9 and 10 illustrate sectional views for explaining a method forreading bit data written in the memory device of FIG. 1;

FIGS. 11 and 12 illustrate sectional views for explaining a process ofwriting bit data “1” in a resistant material by switching, in a memorydevice of FIG. 1; and

FIGS. 13 and 14 are graphs showing changes in current density versustime when bit data are read.

DETAILED DESCRIPTION OF THE INVENTION

Korean Patent Application No. 2002-39988, filed on Jul. 7, 2002, andentitled: “Memory Device Having a Transistor and One Resistant Elementas a Storing Means and Method For Driving the Device” is incorporated byreference herein in its entirety.

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. The invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. It will also be understood that when a layer is referred to asbeing “on” another layer or substrate, it can be directly on the otherlayer or substrate, or intervening layers may also be present. Further,it will be understood that when a layer is referred to as being “under”another layer, it can be directly under, and one or more interveninglayers may also be present. In addition, it will also be understood thatwhen a layer is referred to as being “between” two layers, it can be theonly layer between the two layers, or one or more intervening layers mayalso be present. Like numbers refer to like elements throughout.

Referring to FIGS. 1 through 5, a memory device according to embodimentsof the present invention will be presented.

First Embodiment

A memory device according to the first embodiment of the presentinvention has a transistor on a semiconductor substrate and a resistorinstead of a conventional capacitor, as a means for storing bit datasuch as “0” or “1”.

Specifically, referring to FIG. 1, in a semiconductor substrate 10 aredefined a region in which the memory device is formed (hereinafterreferred to as “active region”) and an region in which a field oxidefilm (or a device separator) is formed for separating the memory device(hereinafter referred to as “field region”). Preferably, thesemiconductor substrate 10 is doped with a p-type conductive impurity.The semiconductor substrate 10 may be an n-type semiconductor substratehaving a p-well, in which the p-type conductive impurity is doped in aregion where the memory device is formed. A field oxide film 11 isformed on the field region of the semiconductor substrate 10. The fieldoxide film 11 is a Locos-type field oxide. The field oxide film 11 maybe substituted by a trench-type oxide film 11A as indicated by thedotted line. A gate electrode 16 is formed on a predetermined region ofthe active region of the semiconductor substrate 10. A gate insulatingfilm 18 exists between the gate electrode 16 and the semiconductorsubstrate 10. There also exist first and second impurity regions 12 and14 which are doped with an n-type conductive impurity to a shallow depthbetween the gate electrode 16 and the field oxide film 11. The firstimpurity region 12 at the left side of the gate electrode 16 is thedrain region of the transistor and the second impurity region 14 at aright side of the gate electrode 16 is the source region of thetransistor. The first and second regions 12 and 14 and the gateelectrode 16 constitute an NPN-type transistor of the memory device. Onthe semiconductor substrate 10, an interlayer insulating film 20 isformed to cover the transistor. A first contact hole 22 exposing thesecond region 14 is formed in the interlayer insulating film 20. Anupper portion of the first contact hole 22 is larger than a lowerportion of the first contact hole 22 in width. The lower portion and alower part of the upper portion of the first contact hole 22 are filledwith a conductive plug 24. The rest of the upper portion of the firstcontact hole 22, namely an uppermost layer having a predeterminedthickness, is filled with a first storing means 26 for storing bit datasuch as “0” or “1”. The first storing means 26 is a resistant material.The resistant material is a material film such as an amorphousdielectric film capable of trapping a charge during a predetermined timerequired for storing data according to a value or a direction of avoltage or current applied from the outside. Preferably, the amorphousdielectric film is a silicon nitride film (Si₃N₄). The amorphousdielectric film may be an aluminum oxide film (Al₂O₃).

It is desirable to change the material of the conductive plug 24according to the material used as the first storing means 26. Forexample, it is desirable that the material of the conductive plug 24 isthe same as the second conductive impurity region 14, i.e., the sourceregion of the transistor, when the first storing means 26 is the siliconnitride film as described above. Therefore, in this case, it isdesirable that the conductive plug 24 is an n-doped poly silicon plug.Further, preferably, the material of the conductive plug 24 is aprecious metal, e.g., gold (Au), when the first storing means 26 is analuminum oxide film.

After that, a first conductive plate 28 is formed on the interlayerinsulating film 20 contacting the entire surface of the first storingmeans 26. The first conductive plate 28 is formed as a line or a pad.Preferably, the material of the first conductive plate 28 is aluminum.

Preferably, the entire thickness of a material layer including theconductive plug 24, the first storing means 26 and the first conductiveplate 28 is about 15 nm.

A member for improving a data storing function of the first storingmeans 26 may be included between the first storing means 26 and theconductive plug 24. The member may be additionally or alternativelyincluded between the first storing means 26 and the first conductiveplate 28. FIGS. 2 and 4 show examples of the foregoing, and aredescribed in greater detail presently.

Referring to FIG. 2, a first material film 30 is used as the member andis included between the conductive plug 24 and the first storing means26.

Referring to FIG. 3, the first material film 30 as the member isincluded between the conductive plug 24 and the first storing means 26.A second material film 32 as the member is included between the firststoring means 26 and the first conductive plate 28.

FIG. 4 shows that the second material film 32 as the member is includedonly between the conductive first storing means 26 and the firstconductive plate 28.

The first material film 30 and the second material film 32 may be formedof an n-type poly silicon film, a p-type poly silicon film or aninsulating film. Here, the insulating film is a silicon oxide film or analuminum oxide film.

Preferably, the thickness between the second impurity region 14 and thefirst conductive plate 28 in a memory device according to the firstembodiment of the present invention is a thickness through which acharge such as an electron, which is trapped in the first storing means26, can tunnel.

Second Embodiment

A second embodiment of the present invention provides a memory deviceexcluding the conductive plug, which connects the transistor and thestoring means.

Specifically, referring to FIG. 5, a transistor is formed on thesemiconductor substrate 10, as in the memory device according to thefirst embodiment, and an interlayer insulating film 20 is formed tocover the transistor. A second contact hole 34, in which a secondimpurity region 14 is exposed, is formed on the interlayer insulatingfilm 20. An insulating film 36 is formed on an exposed region of thesecond impurity region 14 through the second contact hole 34.Preferably, the insulating film 36 is a silicon oxide film formed by anatural oxidation of the region of the second impurity region 14 that isexposed through the second contact hole 34. However, another insulatingfilm may be used instead of the silicon oxidation film provided thecharge can tunnel directly through the insulating film. A second storingmeans 38, which functions similarly to the first storing means 26 of thefirst embodiment illustrated in FIGS. 1–4, is formed on the interlayerinsulating film 20. The second storing means 38 is formed inside thesecond contact hole 34 to be on the inner sidewalls and bottom of thesecond contact hole 34 and to contact the entire surface of theinsulating film 36. Similar to the first storing means 26 of the firstembodiment, it is desirable that the second storing means 38 is aresistant material for storing bit data such as “0” or “1”. Theresistant material is the same as that of the first embodiment, andtherefore, the corresponding description will be omitted. A secondconductive plate 40 is formed on the second storing means 38. Thematerial of the second conductive plate 40 may be aluminum, as in thecase of the first storing means 26 of the first embodiment.

Next, a method for driving memory devices according to embodiments ofthe present invention will be described. For convenience, a method fordriving the memory device according to the first embodiment of thepresent invention will be described. However, the method may also beapplied to the memory device according to the second embodiment of thepresent invention.

Writing Bit Data

Referring to FIG. 1, the storing means 26 is initialized to be suitablefor storing bit data, before the bit data “0” or “1” is written in thefirst storing means 26. This process is called “forming.” During theforming process, charges of the first storing means 26, e.g., electrons,are trapped therein. In the forming process, the transistor remains ONand a forming voltage is applied to the first impurity region 12 througha bit line (not shown). After “forming” the first storing means 26, theconductivity of the first storing means 26 becomes high due to theelectrons trapped in the resistant material thereof. The electronstrapped in the first storing means 26 by the forming process dissipatenaturally over time.

Therefore, as shown in FIG. 6, a gate voltage (Vg) is applied to thegate electrode 16 to keep the transistor ON. At this time, a bit linevoltage (Vb) is applied to the first impurity region 12 through the bitline (not shown). After that, a plate voltage (Vb/2) is applied to thefirst conductive plate 28 to again trap electrons in the first storingmeans 26. Many electrons are trapped in the first storing means 26 afterthe forming process by charging the first storing means 26, as describedabove. The foregoing state is regarded as a state in which the bit data“0” is written to the first storing means 26. When the bit data “0” iswritten, the first storing means 26 becomes highly conductive due to theelectrons trapped therein. Therefore, the resistance of the firststoring means 26 is reduced.

In the meantime, the electrons trapped in the first storing means 26after the forming process are released naturally from the first storingmeans 26 over time. Therefore, after enough time has passed since theforming, a majority of trapped electrons are naturally discharged fromthe first storing means 26. Accordingly, the resistance of the firststoring means 26 increases. When the resistance of the first storingmeans becomes high, the state of the first storing means 26 is regardedas a state in which the bit data “1” is written. However, after theforming is completed, it may take more time than a usual bit datawriting time for the trapped electrons to be naturally discharged fromthe first storing means 26. Therefore, in order to drive the memorydevice fast, it is desirable for the trapped electrons to be dischargedfrom the first storing means 26 as quickly as possible.

To this end, as shown in FIG. 7, the gate voltage (Vg) is applied to thegate electrode 16 to keep the transistor ON and the plate voltage (Vb/2)is applied to the first conductive plate 28. Then, a voltage of 0 volts0[V] is applied to the first impurity region 12 through the bit line(not shown). As a result, the electrons trapped in the first storingmeans 26 are discharged rapidly, the conductivity of the first storingmeans 26 becomes low, and the resistance thereof becomes high, similarto the state of the first storing means 26 prior to the forming thereof.When the first storing means 26 is in a state of high resistance (lowconductivity), the bit data “1” is written.

However, the bit data “1” may be written to the first storing means 26by a switching method, instead of by discharging the trapped electrons.

Specifically, as shown in FIG. 11, the gate voltage (Vg) is applied tothe gate electrode 16 to keep the transistor ON. In this state, aswitching voltage (Vs) is applied to the first conductive plate 28. As aresult, a resistance of the first storing means 26 becomes high as thetrapped electrons in the first storing means 26 are discharged.Accordingly, the first storing means 26 is in a state where the bit data“1” is written. This switching method of writing the bit data “1” isfaster than the method of writing only by discharging trapped electrons.

Another method for writing the bit data “1” is illustrated in FIG. 12.In the method illustrated in FIG. 12, a plate voltage (Vb/2) is appliedto the first conductive plate 28, and a voltage (Vb′) different from thebit line voltage (Vb) of FIG. 6 is applied to the first impurity region12 through the bit line (not shown) to change a resistance value of thefirst storing means 26. In this method, the voltage (Vb′) is variedaccording to the material of the first storing means 26. For example,when the first storing means 26 is a silicon nitride film (Si₃N₄), it isdesirable that the absolute value of the voltage (Vb′) is opposite tothe bit line voltage (Vb) of FIG. 6. When the first storing means 26 isan aluminum oxide film (Al₂O₃), it is desirable that the absolute valueof the voltage (Vb′) is different from the absolute value of the bitline voltage (Vb) of FIG. 6.

In the meantime, in writing the bit data “0”, when the gate voltage (Vg)is not applied to the gate electrode 16 after writing the bit data “0”,as shown in FIG. 8, an open circuit is formed by the plate voltage(Vb/2) applied to the first conductive plate 28. As a result, the timethe electrons trapped in the first storing means 26 are retained in thefirst storing means 26 becomes much longer. Therefore, a time necessaryfor writing the bit data “0” in the first storing means 26 andrecharging the first storing means 26 to retain the data, i.e., arefresh period, becomes long.

In the description above, a state in which electrons are trapped in thefirst storing means 26 is regarded as a state in which the bit data “0”is written to the first storing means 26, and a state in which theresistance of the first storing means 26 is high due to discharging ofthe trapped electrons therefrom is regarded as a state in which the bitdata “1” is written to the first storing means 26.

However, bit data writing may proceed in a way contrary to thatdescribed above. In other words, the state in which the electrons aretrapped in the first storing means 26 may be regarded as a state inwhich the bit data “1” is written to the first storing means 26, and thestate in which the resistance of the first storing means 26 is high dueto discharging of trapped electrons therefrom may be regarded as a statein which the bit data “0” is written to the first storing means 26.

Bit Data Reading

Referring to FIGS. 9 and 10, the bit data written in the first storingmeans 26 may be read by the following two methods.

In the first method, a current flowing through the first storing means26 as the trapped electrons are discharged therefrom may be read by asense amplifier 42 connected to the first impurity region 12 through thebit line (not shown).

In the second method, a current flowing through the resistant materialof the first storing means 26 may be measured by the sense amplifier 42.

In either method, the current measured by the sense amplifier 42 whenreading the bit data “0” is higher than the current measured whenreading the bit data “1”. By comparing the values the currents, it ispossible to determine whether the bit data read from the first storingmeans 26 is “0” or “1”.

FIG. 9 illustrates a sectional view of the memory device of FIG. 1 whilereading a bit data written in the first storing means 26 by the firstmethod described above. In the first method, the first impurity region12 is connected to the sense amplifier 42 through the bit line (notshown) and the gate voltage (Vg) is applied to the gate electrode 16 tokeep the transistor ON. At the same time, the plate voltage (Vb/2) isapplied to the first conductive plate 28. When electrons are trapped inthe first storing means 26, that is, when the bit data “0” is written,the trapped electrons begin to be discharged from the first storingmeans 26. Therefore, a current flows through the sense amplifier 42, andthe sense amplifier 42 measures the current. When the bit data “1” iswritten in the first storing means 26, it is possible to know which bitdata is written in the first storing means 26 by comparing the currentvalues measured by the sense amplifier 42.

FIG. 10 illustrates a sectional view of the memory device of FIG. 7while reading a bit data written in the first storing means 26 by thesecond method described above. In the second method, the first impurityregion 12 is connected to the sense amplifier 42 through the bit lineand the gate voltage (Vg) is applied to the gate electrode 16 to keepthe transistor ON. At the same time, a reading voltage (Vr) is appliedto the first conductive plate 28. Here, it is desirable that the readingvoltage (Vr) is lower than a writing voltage, and that the readingvoltage (Vr) does not allow too many electrons to be discharged from thefirst storing means 26. By such voltage application, when the bit data“0” is written in the first storing means 26, the conductivity of thefirst storing means 26 is high and a current i flows from the firstconductive plate 28 to the first impurity region 12, a channel region(not shown) under the gate electrode 16, the second impurity region 14of the conductive plug 24, and the first storing means 26. The value ofthe current i is measured by the sense amplifier 42.

In the meantime, when the bit data “1” is written in the first storingmeans 26, the conductivity of the first storing means 26 is very low,and the resistance of the first storing means 26 is very high. As aresult, the value of the current i when reading the bit data “1” is muchsmaller than the value of the current i when reading the bit data “0”.Therefore, by comparing the values of the currents, it is possible todetermined whether the data bit written in the first storing means 26 is“0” or “1”.

The reading voltage (Vr) and the value of the current measured using thesense amplifier 42 may respectively vary according to a laminationstructure formed on the second impurity region 14 of the semiconductorsubstrate 10. FIGS. 13 and 14 show variations of current density versustime for different stack structures.

FIG. 13 shows a variation of current density versus time when the bitdata “0” and the bit data “1” are read and when the conductive plug 24,the first storing means 26 formed of Si₃N₄ and first conductive plate 28formed of Al are sequentially stacked. FIG. 14 shows a variation ofcurrent density versus time when a silicon oxide film is furtherincluded between the conductive plug 24 and the first storing means 26.

The result shown in FIG. 13 illustrates a case where the reading voltage(Vr) is −8V, and the result shown in FIG. 14 illustrates a case wherethe reading voltage (Vr) is −5V. When a silicon oxide film is furtherincluded between the conductive plug 24 and the first storing means 26,the reading voltage (Vr) is lower than when the silicon oxide film isnot included.

The methods for reading bit data described above may be appliedregardless of the method used to write the bit data.

As described above, in the memory device according to the presentinvention, a memory cell structure may be simplified and a volume of thememory cell may be reduced when compared with, a capacitor, which is aconventional storing means, by using a thin resistant material as thestoring means. Therefore, the degree of integration of the memory devicemay be enhanced. Further, a structure of the resistant material used forthe storing means is simple compared to that of a conventionalcapacitor. As a result, it is possible to simplify the manufacturingprocess. Additionally, electrons can be retained in the resistantmaterial for a longer period of time, permitting a longer intervalbetween refresh operations. Therefore, it is possible to reduce thecurrent consumption of the memory device.

Preferred embodiments of the present invention have been disclosedherein and, although specific terms are employed, they are used and areto be interpreted in a generic and descriptive sense only and not forpurpose of limitation. Accordingly, it will be understood by those ofordinary skill in the art that various changes in form and details maybe made without departing from the spirit and scope of the presentinvention as set forth in the following claims.

For example, those skilled in the art may use other resistant materialsinstead of a silicon nitride film or an aluminum oxide film for thestoring means. In addition, the insulating film included between theconductive plug and the storing means, and the insulating film includedbetween the storing means and the conductive plug, may be substitutedwith a multi-layered insulating layer. Further, a third material film,which functions as the first material film or the second material film,may be further included between the second storing means and the secondconductive plate. Therefore, the scope of the invention should bedetermined, without departing from the spirit and principles thereof, bythe appended claims and their equivalents.

1. A method for writing bit data of a memory device that includes asemiconductor substrate; an NPN-type transistor formed on thesemiconductor substrate; an interlayer insulating film formed on thesemiconductor substrate to cover the transistor, in which a contact holeexposing a source region of the transistor is formed; a conductive plugfilling the contact hole; a resistant material in which a bit data “0”or “1” is written formed on the conductive plug; and a conductive plateformed on the interlayer insulating film to be contacted with theresistant material, the method comprising: initializing the resistantmaterial; and charging the resistant material to write the bit data “0”or “1” therein.
 2. The method as claimed in claim 1, wherein theconductivity of the resistant material is enhanced by a forming process.3. The method as claimed in claim 2, wherein a forming voltage isapplied to a drain region of the transistor.
 4. The method as claimed inclaim 1, wherein the transistor is on, a bit line voltage (Vb) isapplied to a drain region of the transistor and a plate voltage (Vb/2)is applied to the conductive plate, to write the bit data “0” or “1” inthe resistant material.
 5. The method as claimed in claim 4, whereinafter the bit data “0” or “1” is written, the transistor is turned offto lengthen a retaining time of the bit data.
 6. A method for writingbit data of a memory device that includes a semiconductor substrate; anNPN-type transistor formed on the semiconductor substrate; an interlayerinsulating film formed on the semiconductor substrate to cover thetransistor, in which a contact hole exposing the source region of thetransistor is formed; a conductive plug filling the contact hole; aresistant material in which a bit data “0” or “1” is written formed onthe conductive plug; and a conductive plate formed on the interlayerinsulating film to be contacted with the resistant material, the methodcomprising: initializing the resistant material; and enhancing theresistance of the resistant material and writing the bit data “0” or “1”therein.
 7. The method as claimed in claim 6, wherein the transistor isturned on and a switching voltage (Vs) is applied to the conductiveplate, to enhance the resistance of the resistant material.
 8. Themethod as claimed in claim 6, wherein a conductivity of the resistantmaterial is enhanced by a forming process.
 9. The method as claimed inclaim 8, wherein a forming voltage is applied to a drain region of thetransistor.
 10. The method as claimed in claim 6, wherein the resistantmaterial is discharged to enhance the resistance of the resistantmaterial.
 11. The method as claimed in claim 10, wherein the transistoris turned on and a plate voltage (Vb/2) is applied to the conductiveplate, to enhance the resistance of the resistant material.
 12. Themethod as claimed in claim 6, wherein the resistant material, as anamorphous dielectric film, is a silicon nitride film or an aluminumoxide film.
 13. The method as claimed in claim 12, wherein when theresistant material is the silicon nitride film, the transistor is turnedon, an opposite voltage to a bit line voltage (Vb) is applied to a drainregion of the transistor and a plate voltage (Vb/2) is applied to theconductive plate, to enhance the resistance of the resistant material.14. The method as claimed in claim 12, wherein when the resistantmaterial is the aluminum oxide film, the transistor is turned on, adifferent voltage from a bit line voltage (Vb) is applied to a drainregion of the transistor and a plate voltage (Vb/2) is applied to theconductive plate, to enhance the resistance of the resistant material.15. A method for reading a written bit data in a memory device thatincludes a semiconductor substrate; an NPN-type transistor formed on thesemiconductor substrate; an interlayer insulating film formed on thesemiconductor substrate to cover the transistor, in which a contact holeexposing a source region of the transistor is formed; a conductive plugfilling the contact hole; a resistant material in which a bit data “0”or “1” is written formed on the conductive plug; and a conductive plateformed on the interlayer insulating film to be contacted with theresistant material, wherein the bit data which is written in theresistant material is read by measuring a current flowing through theresistant material and reading the written bit data in the resistantmaterial.
 16. The method as claimed in claim 15, wherein after a senseamplifier is connected to a drain region of the transistor, thetransistor is turned on and a reading voltage (Vr) is applied to theconductive plate to measure the current flowing through the resistantmaterial.